This invention relates to the field of gasification of carbon products of various origins such as coals, lignites, peats, etc., and relates more particularly to a new means making it possible to assure desulfurization of the resulting gases with molten mineral baths, while achieving a substantial saving of materials and energy and allowing the production of mineral compositions having good hydraulic characteristics and pig irons that can be used in metallurgical circuits.
For some years studies have been made of processes for gasification of carbon products, such as coals, that are more or less rich in aches and sulfur. In these processes, problems arise in eliminating the ashes and especially trapping the sulfur to obtain gases rid of this polluting element.
According to several techniques, the ashes from the coal are obtained in the liquid state and are treated like any molten mineral material, e.g., by casting in pits with slow cooling, by hydraulic granulation, by pelletizing, etc.
Efforts to assure as complete a trapping as possible of the sulfur and sulfur compounds that form during coal gasification have led to modifying the chemical composition of the molten ashes to give them a desulfurizing power they do not normally possess due to their essentially being made up of silica, and alumina, acid elements. The additions essentially have a base of materials containing CaO and MgO, such as limestone flux, dolomite, limes and magnesium lime.
By adjusting these addition products as a function of the chemical composition of the ashes, it is possible to obtain molten mineral materials having the same physical and chemical properties as metallurgical slags and particularly having the power to retain sulfur in the form of sulfides such as CaS, FeS and MnS soluble in the molten silicates.
Since coal gasification is normally performed in a reducing medium to obtain a gas mixture rich in CO, the iron oxides going into the composition of the ashes are reduced to the metallic state and yield a pig iron that accumulates in the lower part of the reactor.
Three types of products are obtained in a gasification process of this type: a gas mixture free of sulfur compounds, a slag made up of calcium silicates and silicoaluminates if the addition has only a lime base and a pig iron coming from the iron contained in the coal ashes.
Addition of the product contributing lime especially is expensive in material used and energy consumed since quarried limestone materials have to be introduced which must ge decarbonated either outside or inside the gasification reactor.
It has now been found that it is possible to use, as the basic addition product, instead of limestone or calcomagnesium materials, metallurgical slags having chemical compositions favorable to trapping sulfur, i.e., having the capacity to dissolve the above mentioned sulfur compounds.
It is known that metallurgical slags, such as blast-furnace, converter, or electric-furnace slags, have chemical compositions that enable them to fix the sulfur present in the solid or liquid charges containing iron which are to be treated in the various steelmaking apparatus. These slags are characterized by a desulfurizing power that depends on the basic elements going into their composition and which are classified as follows in descending order: K.sub.2 O, Na.sub.2 O, CaO, MgO, BaO, FeO, MnO. The elements are found in variable proportions depending on the nature of the metallurgical slags considered.
Blast-furnace slag comprises mainly CaO, MgO and a slight percent of alkalines.
Converter and electric furnace slags essentially contain CaO, FeO, MnO and MgO. These slags have much higher basicity indices than blast furnace slags and have the feature of also having more iron and manganese content.
In some cases, steel furnace slags are not entirely recycled in the metallurgical line and are put to less upgrading uses for iron and manganese principally (soil improvement, road aggregate, fill, etc.).